Plating method

ABSTRACT

Methods of depositing layers of copper that selectively incorporate certain impurities are provided. Such copper layers reduce stress-induced void formation in wide copper lines under vias.

The present invention relates generally to the field of metal plating.In particular, the present invention relates to the electrodeposition ofcopper.

Copper is used in the manufacture of many electronic devices. Forexample, in the manufacture of integrated circuits copper damasceneprocesses (including dual damascene) involve the formation of inlaidcopper wiring patterns with the simultaneous formation of viaconnections between metal layers. In such processes, the copper isdeposited electrolytically using direct current.

The purity of the electrolytically deposited copper becomes moreimportant as the size of the electronic devices shrink. High levels ofimpurities in small copper deposits will increase the resistivity of thecopper. Accordingly, the trend in the industry is toward copperelectroplating baths that provide purer copper deposits in order toreduce the resistivity of the deposits.

Stress-induced voiding occurs in copper deposits in dual damascenestructures where voids are formed under vias that connect to wide metallines. Such voiding leads to failures in the device. One theoryattributes the formation of such voiding to vacancies that develop inthe copper deposits when the copper is not properly annealed. See, forexample, E. T. Ogawa et al., Stress-Induced Voiding Under Vias Connectedto Wide Cu Metal Lines, IEEE International Reliability Physics SymposiumProceedings (2002), 40^(th), pp 312-321, which discusses the formationof voids under vias due to stress. Regardless of how such voidingoccurs, the use of higher purity copper in the wide metal linesexacerbates the formation of such voiding. There is a need in theindustry for high purity copper deposits that do not form stress-inducedvoids.

It has been surprisingly found that impurities can be selectivelyincorporated into copper metal lines during electroplating of thecopper. Such selective incorporation of impurities in wide metal linesreduces the formation of stress-induced voids under vias connected tosuch metal lines.

In one embodiment, the present invention provides a method of depositingcopper including the steps of: a) contacting an electronic devicesubstrate having apertures with a copper electroplating bath including asource of copper ions, an electrolyte, and a disulfide-containingaccelerator; b) depositing a layer of copper in the apertures using aduty cycle including 1) applying a first current density for a firstperiod to electrochemically reduce the disulfide-containing acceleratorto a thiol compound at a copper surface; and 2) applying a secondcurrent density for a second period; and c) repeating step b) until adesired copper deposit is obtained; wherein the second current densityis less than the first current density. The present method is useful forincorporating impurities at a desired level within the copper deposit.In particular, the present invention is useful in the manufacture ofintegrated circuits, and more specifically in the deposition of widemetal lines in the manufacture of integrated circuits.

FIG. 1 is a secondary ion mass spectrogram showing impurity levels as afunction of copper film depth for a prior art process.

FIG. 2 is a secondary ion mass spectrogram showing impurity levels as afunction of copper film depth for a prior art process.

FIG. 3 is a secondary ion mass spectrogram showing impurity levels as afunction of copper film depth for the process of the invention.

As used throughout the specification, the following abbreviations shallhave the following meanings: nm=nanometers; g/L=grams per liter; mA/cm²=milliamperes per square centimeter; μm=micron=micrometer; ppm=parts permillion, mL/L=milliliter/liter;° C.=degrees Centigrade; sec.=seconds;msec.=milliseconds; g=grams; DC =direct current; Hz=Hertz; andÅ=Angstroms.

As used throughout the specification, “feature” refers to the geometrieson a substrate. “Apertures” refer to recessed features, such as vias andtrenches. As used throughout this specification, the term “plating”refers to copper electroplating, unless the context clearly indicatesotherwise. “Deposition” and “plating” are used interchangeablythroughout this specification. “Defects” refer to surface defects of acopper layer, such as protrusions and pits, as well as defects within acopper layer, such as voids. “Wide metal lines” refers to metal lineshaving a width of >1 μm. The terms “layer” and “film” are usedinterchangeably and refer to a metal deposit, particularly a copperdeposit, unless the context clearly indicates otherwise.

The term “alkyl” includes linear, branched and cyclic alkyl.“Accelerator” refers to an organic additive that increases the platingrate of a metal during electroplating. “Suppressors” (also known as“carriers”) refer to organic additives that suppress the plating rate ofa metal during electroplating. “Leveler” refers to an organic additivethat is capable of providing a substantially planar metal layer. Theterms “leveler” and “leveling agent” are used interchangeably throughoutthis specification. The term “halide” refers to fluoride, chloride,bromide and iodide. As used herein, “duty cycle” means the relationshipbetween the time period of high current density and the time period oflow current density. A 75% duty cycle means that for a given time theratio of time periods of high to low current density is 3:1 (or that thehigh current density is applied for 75% of the time and the low currentdensity is applied for 25% of the time).

The indefinite articles “a” and “an” are intended to include both thesingular and the plural. All percentages and ratios are by weight unlessotherwise indicated. All ranges are inclusive and combinable in anyorder except where it is clear that such numerical ranges areconstrained to add up to 100%.

A wide variety of electronic device substrates may be plated with copperaccording to the present invention. Suitable substrates include, withoutlimitation: printed circuit board substrates, integrated circuitsubstrates such as wafers used in the manufacture of integratedcircuits, electronic packages such as lead frames and electronicinterconnects such as wafer bumps; and optoelectronic device substratessuch as hermetic sealing layers.

A wide variety of copper electroplating baths may be used with thepresent invention. Copper electroplating baths typically contain asource of copper ions, an electrolyte, a source of chloride ions, and adisulfide-containing accelerator. More typically, organic additives suchas a suppressor are added to the copper electroplating baths. The copperelectroplating baths may optionally contain a leveler.

Typical sources of copper ions are any copper compounds that are solublein the electroplating bath. Suitable copper compounds include, but arenot limited to, copper salts such as copper sulfate, copper persulfate,copper halide, copper chlorate, copper perchlorate, copperalkanesulfonate such as copper methanesulfonate, copper alkanolsulfonate, copper arylsulfonate, copper fluoroborate, cupric nitrate,copper acetate, and copper citrate. Copper sulfate is preferred.Mixtures of copper compounds may be used. Such sources of copper ionsare generally commercially available.

The source of copper ions may be used in the present electroplatingbaths in a relatively wide concentration range. Typically, the copperion source is present in an amount sufficient to provide an amount ofcopper ion of 10 to 80 g/L in the plating bath. More typically, theamount of copper source provides 15 to 65 g/L of copper ions in theplating bath. The copper plating bath may also contain amounts of otheralloying elements, such as, but not limited to, tin, zinc, indium,antimony, and the like. Such alloying elements are added to theelectroplating baths in the form of any suitable bath-solution salt.Thus, the copper electroplating baths useful in the present inventionmay deposit copper or copper alloy.

The electrolyte may be alkaline or acidic and is typically acidic. Anyacid which is compatible with the copper compound may be used in thepresent invention. Suitable acids include, but are not limited to:sulfuric acid, acetic acid, fluoroboric acid, nitric acid, sulfamicacid, phosphoric acid, hydrogen halide acids such as hydrochloric acid,alkanesulfonic acids and arylsulfonic acids such as methanesulfonicacid, toluenesulfonic acid, phenolsulfonic acid and benzenesulfonicacid, and halogenated acids such as trifluoromethylsulfonic acid andhaloacetic acid. Typically the acid is sulfuric acid, alkanesulfonicacid or arylsulfonic acid. Mixtures of acids may be used. In general,the acid is present in an amount to impart conductivity to a bathcontaining the acidic electrolyte composition. Typically, the pH of theacidic electrolyte of the present invention has a value of less than 7,and more typically less than 2. Exemplary alkaline electroplating bathsuse pyrophosphate as the electrolyte, although other electrolytes may beemployed. It will be appreciated by those skilled in the art that the pHof the electrolyte may be adjusted by any known methods, if necessary.

The total amount of acid electrolyte used in the present electroplatingbaths may be from 0 to 100 g/L, and typically from 0 to 60 g/L, althoughhigher amounts of acid may be used for certain applications, such as upto 225 g/L or even 300 g/L. It will be appreciated by those skilled inthe art that by using copper sulfate, a copper alkanesulfonate or acopper arylsulfonate as the copper ion source, an acidic electrolyte canbe obtained without any added acid.

A wide variety of disulfide-containing accelerators may be employed inthe present copper electroplating baths. Such accelerators may be usedalone or as a mixture of two or more. In general, thedisulfide-containing accelerators have a molecular weight of 5000 orless and more typically 1000 or less. Disulfide-containing acceleratorsthat also have sulfonic acid groups are generally preferred,particularly compounds that include a group of the formulaR′—S—S—R—SO₃X, where R is an optionally substituted alkyl (which includecycloalkyl), optionally substituted heteroalkyl, optionally substitutedaryl group, or optionally substituted heteroalicyclic; X is hydrogen ora counter ion such as sodium or potassium; and R′is hydrogen or anorganic residue, such as a group of the formula —R—SO₃X or a substituentof a larger compound. Typically alkyl groups will have from 1 to 16carbons, more typically 1 to 8 or 12 carbons. Heteroalkyl groups willhave one or more hetero (N, O or S) atoms in the chain, and typicallyhave from 1 to 16 carbons, more typically 1 to 8 or 12 carbons.Carbocyclic aryl groups are typical aryl groups, such as phenyl andnaphthyl. Heteroaromatic groups also will be suitable aryl groups, andtypically contain 1 to 3 of one or more of N, O and S atoms and 1 to 3separate or fused rings and include, e.g., coumarinyl, quinolinyl,pyridyl, pyrazinyl, pyrimidyl, furyl, pyrrolyl, thienyl, thiazolyl,oxazolyl, oxidizolyl, triazole, imidazolyl, indolyl, benzofuranyl, andbenzothiazol. Heteroalicyclic groups typically will have 1 to 3 of oneor more of N, O and S atoms and from 1 to 3 separate or fused rings andinclude, e.g., tetrahydrofuranyl, thienyl, tetrahydropyranyl,piperdinyl, morpholino, and pyrrolindinyl. Substituents of substitutedalkyl, heteroalkyl, aryl or heteroalicyclic groups include, e.g., C₁₋₈alkoxy; C₁₋₈ alkyl, halogen such as F, C1 and Br; cyano; and nitro.

More specifically, useful disulfide-containing accelerators includethose of the following formulae; XO₃S—R—S—S—R—SO₃ X andXO₃S—Ar—S—S—Ar—SO₃X, wherein R in the above formulae is an optionallysubstituted alkyl group, and typically is an alkyl group having from 1to 6 carbon atoms, more typically is an alkyl group having from 1 to 4carbon atoms; Ar is an optionally substituted aryl group such asoptionally substituted phenyl or naphthyl; and X is hydrogen or asuitable counter ion such as sodium or potassium. Exemplarydisulfide-containing accelerators include, without limitation,bis-sulfopropyl disulfide and bis-sodium-sulfopropyl disulfide.

Optionally, an additional accelerator that does not contain a disulfidegroup may be used in combination with the present disulfide-containingaccelerator. Typical additional accelerators are sulfur-containing andcontain one or more sulfur atoms and may be, without limitation, thiols,mercaptans, sulfides, disulfides and organic sulfonic acids. In oneembodiment, such additional accelerator compound has the formulaXO₃S—R—SH, wherein R is an optionally substituted alkyl group, andtypically is an alkyl group having from 1 to 6 carbon atoms, moretypically is an alkyl group having from 1 to 4 carbon atoms and X ishydrogen or a suitable counter ion such as sodium or potassium.

Exemplary additional accelerators include, without limitation,N,N-dimethyl-dithiocarbamic acid-(3-sulfopropyl)ester;3-mercapto-propylsulfonic acid-(3-sulfopropyl)ester;3-mercapto-propylsulfonic acid (sodium salt); carbonicacid-dithio-o-ethylester-s-ester with 3-mercapto-1 -propane sulfonicacid (potassium salt); 3-(benzthiazolyl-s-thio)propyl sulfonic acid(sodium salt); pyridinium propyl sulfobetaine;1-sodium-3-mercaptopropane-1-sulfonate; sulfoalkyl sulfide compoundsdisclosed in U.S. Pat. No. 3,778,357; the peroxide oxidation product ofa dialkyl amino-thiox-methyl-thioalkanesulfonic acid; and combinationsof the above. Additional suitable accelerators are also described inU.S. Pat. Nos. 3,770,598; 4,374,709; 4,376,685; 4,555,315; and4,673,469.

The amount of the disulfide-containing accelerators present in a freshlyprepared copper electroplating bath is typically from 0.1 to 1000 ppm.More typically, the disulfide-containing accelerator compounds arepresent in an amount of from 0.5 to 300 ppm, still more typically from 1to 100 ppm, and even more typically from 2 to 50 ppm. Any additionalaccelerators present in the copper electroplating bath is used theamounts described for the disulfide-containing accelerators.

In general, the copper electroplating baths also contain water. Thewater may be present in a wide range of amounts. Any type of water maybe used, such as distilled, deionized or tap.

It will be appreciated by those skilled in the art that one or moreother components may be added to the copper electroplating baths of thepresent invention, such as, e.g., suppressors, levelers, halide ions,and other alloying materials.

Any suppressor may optionally be used in the present electroplatingbaths. Suppressors, as used throughout this specification, refer to anycompounds that suppress the plating rate of copper as compared to bathswithout such suppressors. Suitable suppressors include polymericmaterials, preferably having heteroatom substitution, particularlyoxygen linkages. In general, suppressors are typically polyethers, suchas, but not limited to, those of the formulaR—O—(CXYCX′Y′O)_(n)Hwherein R is an aryl, alkyl or alkenyl group containing from 2 to 20carbons; X, Y, X′, and Y′ are each independently hydrogen, alkyl, suchas methyl, ethyl or propyl, aryl such as phenyl, and aralkyl such asbenzyl; and n is an integer from 5 to 100,000. It is preferred that oneor more of X, Y, X′ and Y′ is hydrogen. More than one suppressor may beused.

Suitable suppressors include, but are not limited to: amines such asethoxylated amines; polyoxyalkylene amines and alkanol amines; amides;poly-glycol type wetting agents such as polyethylene glycols,polyalkylene glycols and polyoxyalkylene glycols; high molecular weightpolyethers; polyethylene oxides such as those having a molecular weightin the range of 1,000 to 100,000; polyoxyalkylene block copolymers;alkylpolyether sulfonates; complexing suppressors such as alkoxylateddiamines; and complexing agents for cupric or cuprous ions such ascitric acid, edetic acid, tartaric acid, potassium sodium tartrate,acetonitrile, cupreine and pyridine.

Particularly useful suppressors include, but are not limited to:ethyleneoxide/propyleneoxide (“EO/PO”) block or random copolymers;ethoxylated polystyrenated phenol having 12 moles of ethyleneoxide(“EO”), ethoxylated butanol having 5 moles of EO, ethoxylated butanolhaving 16 moles of EO, ethoxylated butanol having 8 moles of EO,ethoxylated octanol having 12 moles of EO, ethoxylated beta-naphtholhaving 13 moles of EO, ethoxylated bisphenol A having 10 moles of EO,ethoxylated sulfated bisphenol A having 30 moles of EO and ethoxylatedbisphenol A having 8 moles of EO.

In general, the suppressor may be added in any amount that providessufficient lateral growth of the copper layer. Typically, the amount ofsuppressor is in the range of 0.001 to 10 g/L, and more typically 0.1 to2.0 g/L.

Levelers may optionally be added to the present electroplating baths. Inone embodiment, a leveler compound is used in the present electroplatingbaths. Such levelers may be used in a wide range of amounts, such asfrom 0.01 to 50 ppm or greater. Examples of suitable leveling agents aredescribed and set forth in U.S. Pat. Nos. 3,770,598; 4,374,709;4,376,685; 4,555,315; 4,673,459; and 6,610,192; and U.S. pat.application Ser. No. 2004/0249177. In general, useful leveling agentsinclude those that contain a substituted amino group such as compoundshaving R—N—R′, where each R and R′is independently a substituted orunsubstituted alkyl group or a substituted or unsubstituted aryl group.Typically the alkyl groups have from 1 to 6 carbon atoms, more typicallyfrom 1 to 4 carbon atoms. Suitable aryl groups include substituted orunsubstituted phenyl or naphthyl. The substituents of the substitutedalkyl and aryl groups may be, for example, alkyl, halo and alkoxy.Sulfur-containing leveling agents may also be used.

More specifically, suitable leveling agents include, but are not limitedto, 1-(2-hydroxyethyl)-2-imidazolidinethione; 4-mercaptopyridine;2-mercaptothiazoline; ethylene thiourea; thiourea; alkylatedpolyalkyleneimine; phenazonium compounds disclosed in U.S. Pat. No.3,956,084; N-heteroaromatic rings containing polymers; quatemized,acrylic, polymeric amines; polyvinyl carbamates; pyrrolidone; andimidazole. An exemplary leveler is1-(2-hydroxyethyl)-2-imidazolidinethione, although other suitablelevelers may be employed.

Other suitable levelers are reaction products of an amine with anepihalohydrin, and preferably epichlorohydrin. Suitable amines include,but are not limited to, primary, secondary or tertiary amines, cyclicamines, aromatic amines and the like. Exemplary amines include, withoutlimitation, dialkylamines, trialkylamines, arylalylamines, diarylamines,imidazole, triazole, tetrazole, benzimidazole, benzotriazole,piperidine, morpholine, piperazine, pyridine, oxazole, benzoxazole,pyrimidine, quinoline, and isoquinoline. Imidazole is the preferredamine. Such amines may be substituted or unsubstituted. By“substituted”, it is meant that one or more of the hydrogens on theamine are replaced by one or more substituent groups, such as alkyl,aryl, alkoxy, halo, and alkenyl. Other suitable reaction products ofamines with epichlorohydrin are those disclosed in U.S. Pat. No.4,038,161 (Eckles et al.). Such reaction products are generallycommercially available, such as from Raschig, or may be prepared bymethods known in the art.

When present, the leveling agents are typically used in an amount of 0.5to 1000 ppm. More typically, the leveling agents are used in an amountof 0.5 to 500 ppm, still more typically from 1 to 250 ppm, and even moretypically from 1 to 50 ppm.

The present copper electroplating baths may optionally contain a halideion, and preferably do contain a halide ion. Chloride and bromide arepreferred halide ions, with chloride being more preferred. Mixtures ofhalide ions may be used. A wide range of halide ion concentrations (if ahalide ion is employed) may be suitably utilized, e.g. from 0 (where nohalide ion employed) to 100 ppm of halide ion in the plating bath, morepreferably from 25 to 75 ppm. Such halides may be added as thecorresponding hydrogen halide acid or as any suitable salt.

The electroplating baths may be prepared by combining the source ofcopper ions, the electrolyte, the disulfide-containing accelerator andany optional components in any order. Typically, the plating baths ofthe present invention may be used at any temperature from 10° to 65° C.or higher. It is preferred that the temperature of the plating baths isfrom 10° to 35° C. and more preferably from 15° to 30° C.

The present plating baths are typically agitated during use. Anysuitable agitation method may be used with the present invention andsuch methods are well-known in the art. Suitable agitation methodsinclude, but are not limited to, air sparging, work piece agitation,impingement, rotation and the like. Such methods are known to thoseskilled in the art.

When the present invention is used to plate an integrated circuitsubstrate, such as a wafer, the wafer may be rotated such as from 1 to150 RPM and the plating solution contacts the rotating wafer, such as bypumping or spraying. In the alternative, the wafer need not be rotatedwhere the flow of the plating bath is sufficient to provide the desiredmetal deposit.

In general, the substrate to be copper plated is contacted with thecopper electroplating bath by a suitable means, such as by immersion orby pumping or spraying. The substrate typically functions as thecathode. An anode is added to the copper plating bath and a potential isapplied.

In one embodiment, the present invention provides a method of depositingcopper including the steps of: a) contacting an electronic devicesubstrate having apertures with a copper electroplating bath including asource of copper ions, an electrolyte, and a sulfur-containing compound;b) depositing a layer of copper in the apertures using a duty cycleincluding 1) applying a first current density for a first period toelectrochemically reduce the disulfide-containing accelerator to a thiolcompound at a copper surface; and 2) applying a second current densityfor a second period; and c) repeating step b) until a desired copperdeposit is obtained; wherein the second current density is less than thefirst current density.

The duty cycle may be repeated at a variety of different frequencies.For example, the duty cycle may be repeated up to multiple times persecond or may take multiple seconds to perform one duty cycle. Theparticular duty cycle chosen will depend upon the size of the apertureto be copper plated, the particular copper electroplating bath used andthe level of impurities desired. Suitable duty cycle frequencies arefrom 0.05 to 10 Hz (or cycles per second) or even higher frequencies maybe used, such as up to 100 Hz. In the manufacture of integrated circuitshaving wide metal lines, a suitable duty cycle has a frequency of 0.1 to10 Hz, more typically from 0.1 to 5 Hz and still more typically from 0.1to 2 Hz, although higher or lower frequencies may suitably be used.

A wide variety of current densities may be used for the first currentdensity. Suitable first current densities are from 1 to 100 mA/cm²although higher or lower current densities may be used. More typically,the first current density is from 5 to 100 mA/cm², and still moretypically from 15 to 90 mA/cm². A particularly suitable range of firstcurrent densities is from 40 to 85 mA/cm^(2 .) A wide variety of currentdensities may be used for the second current density, provided that thesecond current density is less than the first current density. Exemplarysecond current densities are from 1 to 50 mA/cm², although higher orlower current densities may be used. More typically, the second currentdensity is from 1 to 35 mA/cm², still more typically from 2 to 25mA/cm², and even more typically from 5 to 10 mA/cm².

While not intending to be bound by theory, it is believed that the firstperiod of high current density reduces the disulfide-containingaccelerator to one or more thiol compounds. Such thiol compounds maycontain one or more thiol groups. It is believed that thedisulfide-containing accelerator is electrochemically reduced at thefreshly growing copper surface to form the thiol compound. Such thiolcompounds are believed to adsorb on the copper surface during therelatively high current density first period. In one embodiment, thefirst period is performed for a time of 0.1 msec. to 10 sec., moretypically from 0.1 msec. to 5 sec., and still more typically from 0.1msec. to 1 sec. Further without wishing to be bound by theory, it isbelieved that the longer the period of relatively low current density,the greater the amount of total impurities incorporated into the copperdeposit. In one theory, but not the only theory, such period ofrelatively low current density allows the copper surface torecrystallize to incorporate any organic material on the copper surface.Thus, the amount of total impurities incorporated into the copperdeposited can be controlled by the choice of second current density andby the time period the substrate is subjected to this current density.

In the present process, the range of amounts of impurities incorporatedin the copper layer as deposited, that is before annealing) may be from1 to 500 ppm for each impurity, such as chloride, sulfur, carbon, oxygenand nitrogen. The total amount of impurities before annealing may be upto a couple of thousand ppm. . Typically, such total impurities are inthe range of from 1 to 500 ppm, more typically from 1 to 300 ppm. Theimpurity levels are determined by Secondary Ion Mass Spectrometry(“SIMS”), which provides a value of ion concentration per unit area, ascompared to an ion implanted standard. The average impurity values areobtained by summing the ppm values from the SIMS analysis for each datapoint for each impurity and then dividing by the total number of datapoints for the depth (in nm) of the copper layer evaluated. The averageimpurity levels throughout the depth of the copper deposit are muchlower than the individual values. For example, an impurity level ofchloride ion by SIMS analysis may show a maximum value of 200 ppm for agiven unit area, where the average chloride ion impurity level may onlybe 5 ppm for the entire copper deposit. In one embodiment, the range ofaverage total impurity level is from 1 to 500 ppm, and more typicallyfrom 1 to 300 ppm.

In integrated circuit manufacture, copper layers are typically annealed.During such annealing step, certain impurities, such as sulfur andoxygen, are typically reduced. Copper layers deposited according to thepresent invention, following annealing, typically have average totalimpurities in the range of 1 to 500 ppm, more typically 1 to 300 ppm,and still more typically from 1 to 250 ppm. In one embodiment, theaverage total impurity level following annealing is from 1 to 100 ppm.

After the desired copper deposit is obtained, an optional furtherplating step may be employed to smooth the surface of the deposit. Suchoptional plating step includes applying a current density for a thirdperiod. A further optional resting step may be included. No current isapplied during the resting portion of the step. In one embodiment, thethird current density is in the range of 20 to 90 mA/cm² and typically30 to 60 mA/cm². In another embodiment, additional plating steps areperformed, such additional steps may include cycling the plating on andoff to smooth the surface of the copper deposit.

The present invention is useful for depositing copper as well as copperalloys such as, but not limited to, copper-silver, copper-tin andtin-copper-silver. The present invention is expected to be beneficial inthe deposition of metals other than copper, such as silver and tin.

An advantage of the present invention is that it provides for thetailoring of doping (impurity) levels in a metal layer, particularly acopper layer, to balance electromigration performance and void stressmigration performance. High purity levels (i.e. low doping levels) areadvantageous from electromigration performance. However, theincorporation of certain levels of impurities may be beneficial for voidstress migration control where small vias land on a wide line.

A further advantage of the present invention is that a single metalplating bath may be used to provide an electronic device having a firstmetal layer having a first purity and a second metal layer having asecond purity, where the purities of the two metal layers are different.In this way, a metal layer can be deposited having a desired level oftotal impurities needed for a specific purpose, such as for control ofvoid stress migration. Accordingly, the present invention provides anelectronic device including a first layer of metal and a second layer ofmetal, wherein the first layer of metal includes total impurities in therange of up to 10 ppm and the second layer of metal includes totalimpurities in the range of 10 to 100 ppm. In one embodiment, the firstand second metal layers are copper. For example, in an integratedcircuit, the first layer of metal may be a via layer, a small line(i.e., a line having a width of ≦1 μm), or a mixture of these and thesecond layer of metal may be a wide line.

EXAMPLE 1-10

A copper plating bath was prepared by combining copper sulfate (40 g/Lof copper ion), sulfuric acid (10 g/L), hydrochloric acid (50 mg/L ofchloride ion), a disulfide-containing sulfonic acid accelerator (10mL/L), an EO/PO copolymer suppressor (5 mL/L), a leveler (3 mL/L) thatis a reaction product of an epoxide and imidazole and water.

Wafers were plated by immersing them individually in the copper platingbath with rotation to cause net mass transport to the wafer surface.Different first and second current densities were used for each wafer.In each case, a rectangular pulsed waveform having a 75% duty cycle wasused. Copper was deposited to approximately 1 μm. After deposition, thewafers were removed from the plating bath, rinsed and dried. The copperdeposits were then analyzed by Secondary Ion Mass Spectrometry (“SIMS”)for total impurity levels and found to contain oxygen, nitrogen,chlorine, sulfur and carbon as impurities. The approximate total amountof impurities (C, N, O, S, Cl) in each deposit is reported in thefollowing table. High Current Low Current Average Total Density DensityFrequency Impurity Level Example (mA/cm²) (mA/cm²) (Hz) (ppm) 1 40 5 111.0 2 40 5 0.5 14.2 3 40 5 0.25 13.6 4 55 5 1 26.4 5 55 5 0.5 21.0 6 555 0.25 20.7 7 65 5 0.25 26.0 8 65 5 0.1 24.4 9 85 5 0.25 38.3 10 85 50.1 35.5

EXAMPLE 11—Comparative

The plating bath of Examples 1-10 was used to deposit copper on a waferusing a DC waveform. The average (C, N, O, S, Cl) impurity level by SIMSanalysis was <5 ppm.

EXAMPLE 12—Comparative

The plating bath of Examples 1-10 was used to deposit a copper film on awafer using a with a current density of 7 mA/cm² for the first 100 nm ofcopper deposited, followed by 40 mA/cm² for approximately the next 900nm of copper deposited. The non-annealed copper deposit (approximately1000 nm thick) was then analyzed for impurity levels using SIMS. Theresults are shown in FIG. 1 and illustrate the principal impurities:carbon, sulfur, chlorine, nitrogen, and oxygen. The plottedconcentration values are not average values but are instead actual datapoints. The increasing levels of impurities observed near 0 nm of copperdeposit depth arose from surface contamination from the additives in theplating bath. The high level oxygen impurity at depths of >800 nm arosefrom the TaO liner used to fabricate the silicon test wafer. These datashow, for example, a maximum concentration of approximately 6 ppm ofchloride ion per unit area in the region of 400-700 nm depth. Theaverage total impurity level of this copper deposit was quite low, i.e., 5 ppm. The average total impurity level was not controlled using thisprocess.

EXAMPLE 13 —Comparative

The plating bath of Examples 1-10 was used to deposit a copper film on awafer using a constant current density of 7 mA/cm² for the entire depthof the copper deposit (approximately 1000 nm). The non-annealed copperdeposit was then analyzed for impurity levels using SIMS. The resultsare shown in FIG. 2 and illustrate the principal impurities: carbon,sulfur, chlorine, nitrogen, and oxygen. The plotted concentration valuesare not average values but are instead actual data points. Theincreasing level of oxygen at depths of >850 nm arose from the TaO linerused to fabricate the silicon test wafer. The very high levels ofimpurities observed between 300 and 450 nm depth arose from a naturalsurface recrystallization phenomenon that results in the incorporationof surface adsorbates from the plating bath and exposure of a freshsurface of copper atoms. This natural cycle of accumulation of surfaceadsorbates followed by recrystallization of the surface layers can berepeated indefinitely if plating is continued at low current density.However, when such recrystallization occurs is not predictable.Accordingly, the average total impurity level cannot be controlled usingsuch natural recrystallization process.

EXAMPLE 14

The plating bath of Examples 1-10 was used to deposit a copper film on awafer using alternating current densities of 5 mA/cm² (for 100 nm ofcopper deposit) and 60 mA/cm² (for 22.5 nm) repeated four times. Thefinal 410 nm of copper deposit was plated at 5 mA/cm². The non-annealedcopper deposit was then analyzed for impurity levels using SIMS. Theresults are shown in FIG. 3 and illustrate the principal impurities:carbon, sulfur, chlorine, nitrogen, and oxygen. The plottedconcentration values are not average values but are instead actual datapoints. The high level of oxygen at depths of >950 nm arose from the TaOliner used to fabricate the silicon test wafer. The very high levels ofother impurities observed between 300 and 950 nm depth arose frominduced surface recrystallization caused by enrichment with surfaceadsorbed thiols that are produced during brief applications for a highcurrent density plating pulse (1 sec, 60 mA/cm²). Pulsed waveforms ofthe present invention can be used, therefore, to incorporate very largelevels of impurities compared with a conventional DC plating process.

1. A method of depositing copper comprising the steps of: a) contactingan electronic device substrate having an aperture with a copperelectroplating bath comprising a source of copper ions, an electrolyte,and a disulfide-containing accelerator; b) depositing a layer of copperin the aperture using a duty cycle comprising 1) applying a firstcurrent density for a first period to electrochemically reduce thedisulfide-containing accelerator to a thiol compound at a coppersurface; and 2) applying a second current density for a second period;and c) repeating step b) until a desired copper deposit is obtained;wherein the second current density is less than the first currentdensity.
 2. The method of claim 1 wherein the copper deposit comprisesfrom 1 to 500 ppm of average total impurities after annealing of thecopper.
 3. The method of claim 2 wherein the impurities comprise one ormore of carbon, oxygen, nitrogen, sulfur and chloride.
 4. The method ofclaim 1 wherein the first period is up to 5 seconds.
 5. The method ofclaim 4 wherein the duty cycle has a frequency of 0.1 to 10 Hz.
 6. Themethod of claim 1 wherein the first current density is in the range of10 to 100 mA/cm².
 7. The method of claim 1 wherein the second currentdensity is in the range of 1 to 20 mA/cm².
 8. The method of claim 1wherein the duty cycle has a ratio of step 1) to step 2) of 1:1 to 10:1.9. An electronic device comprising a first layer of metal and a secondlayer of metal, wherein the first layer of metal comprises average totalimpurities in the range of up to 10 ppm and the second layer of metalcomprises average total impurities in the range of 10 to 100 ppm. 10.The electronic device of claim 10 wherein the first and second metallayers are copper.